The present invention relates to material handling devices that lift and lower loads as a function of operator-applied force.
The device described here is different from manual material handling devices currently used by auto-assembly and warehouse workers. Initial research generally shows three types of material handling devices are currently available on the market.
A class of material handling devices called balancers consists of a motorized take-up pulley, a line that wraps around the pulley as the pulley turns, and an end-effector that is attached to the end of the line. The end-effector has components that connect to the load being lifted. The pulley""s rotation winds or unwinds the line and causes the end-effector to lift or lower the load connected to it. In this class of material handling systems, an actuator generates an upward line force that exactly equals the gravity force of the object being lifted so that the tension in the line balances the object""s weight. Therefore, the only force the operator must impose to maneuver the object is the object""s acceleration force. This force can be substantial if the object""s mass is large. Therefore, a heavy object""s acceleration and deceleration is limited by the operator""s strength.
There are two ways of creating a force in the line so that it exactly equals the object weight. First, if the system is pneumatically powered, the air pressure is adjusted so that the lift force equals the load weight. Second, if the system is electrically powered, the right amount of voltage is provided to the amplifier to generate a lift force that equals the load weight. The fixed preset forces of balancers are not easily changed in real time, and therefore these types of systems are not suited for maneuvering of objects of various weights. This is true because each object requires a different bias force to cancel its weight force. This annoying adjustment must be done either manually by the operator or electronically by measuring the object""s weight. For example, the pneumatic balancers made by Zimmerman International Corporation or Knight Industries are based on the above principle. The air pressure is set and controlled by a valve to maintain a constant load balance. The operator has to manually reach the actuator and set the system to a particular pressure to generate a constant tensile force on the line. The LIFTRONIC System machines made by Scaglia also belong in the family of balancers, but they are electrically powered. As soon as the system grips the load, the LIFTRONIC machine creates an upward force in the line which is equal and opposite to the weight of the object being held. These machines may be considered superior to the Zimmerman pneumatic balancers because they have an electronic circuit that balances the load during the initial moments when the load is grabbed by the system. As a result, the operator does not have to reach the actuator on top and adjust the initial force in the line. In this system, the load weight is measured first by a force sensor in the system. While this measurement is being performed, the operator should not touch the load, but instead should allow the system to find the object""s weight. If the operator does touch the object, the force reading will be incorrect. As a result, the LIFTRONIC machine then creates an upward line force that is not equal and opposite to the weight of the object being held. Unlike the assist device of this application, balancers do not give the operator a physical sense of the force required to lift the load. Also, unlike the device of this application, balancers can only cancel the object""s weight with the line""s tension and are not versatile enough to be used in situations in which load weights vary.
The second class of material handling device is similar to the balancers described above, but the operator uses an intermediary device such as a valve, push-button, keyboard, switch, or teach pendent to adjust the lifting and lowering speed of the object being maneuvered. For example, the more the operator opens the valve, the greater will be the speed generated to lift the object. With an intermediary device, the operator is not in physical contact with the load being lifted, but is busy operating a valve or a switch. The operator does not have any sense of how much she/he is lifting because his/her hand is not in contact with the object. Although suitable for lifting objects of various weights, this type of system is not comfortable for the operator because the operator has to focus on an intermediary device (i.e., valve, push-button, keyboard, or switch). Thus, the operator pays more attention to operating the intermediary device than to the speed of the object, making the lifting operation rather unnatural.
The third class of material handling device use end-effectors equipped with force sensors or motion sensors. These devices measure the human force or motion and based on this measurement vary the speed of the actuator. An example of such a device is U.S. Pat. No. 4,917,360 to Yasuhiro Kojima. With this and with similar devices, if the human pushes upward on the end-effector the pulley turns and lifts the load; and if the human pushes downward on the end-effector, the pulley turns and lowers the load. A problem occurs when the operator presses downward on the end-effector to engage the load with the suction cups, the controller and actuator interpret this motion as an attempt to lower the load. As a result, the actuator causes the pulley to release more line than necessary, creating xe2x80x9cslackxe2x80x9d in the cable. Hereinafter the term xe2x80x9cslackxe2x80x9d should be interpreted as meaning an excessive length of line but should not be construed as including instances where the line is simply not completely taut. A slack line may wrap around the operator""s neck or hand. After the slack is produced in the line by this or other circumstances, when the operator pushes upwardly on the handle, the slack line can become tight around the operator""s neck or hand creating deadly injuries. Because slack can occur even when suction cups are not used as the load gripping means, for safe operation it is important to prevent slack at all times. During fast maneuvers workers can accidentally hit the loads they intend to lift or their surrounding environment (e.g. conveyor belts) with the bottom of the end-effector. In palletizing tasks, the workers quite often use the bottom of the end-effector to fine tune the locations of a box that is not well placed. These occurrences will cause slack in the line since the operator pushes downwardly on the end-effector handle to situate a box, while the end-effector is constrained from moving downwardly. In general, slack in the line can be dangerous for the operator and others the same work environment. The manual material handling device of my invention never creates slack in the line.
The force sensor devices of this class also fail to give an operator a realistic sense of the weight of the load being lifted. This can lead to unnatural and possibly dangerous load maneuvers.
The assist device of this application solves the above problems associated with the three classes of material handling devices. The hoist of this invention includes an end-effector to be held by a human operator; an actuator such as an electric motor; a computer or other type of controller for controlling the actuator; and a line, cable, chain, rope, wire or other type of line for transmitting a tensile lifting force between the actuator and the end-effector. Hereinafter the term xe2x80x9cliftingxe2x80x9d should be interpreted as including both upward and downward movements of a load. The end-effector provides an interface between the human operator and an object that is to be lifted. A force transfer mechanism such as a pulley, drum or winch is used to apply the force generated by the actuator to the line that transmits the lifting force to the end-effector.
A signal representing the vertical force imposed on the end-effector by the human operator, as measured by a sensor, is transmitted to the controller that is associated with the actuator. In operation, the controller causes the actuator to rotate the pulley and move the end-effector appropriately so that the human operator only lifts a preprogrammed small proportion of the load force while the remaining force is provided by the actuator. Therefore, the actuator assists the operator during lifting movements in response to the operator""s hand force. Moreover, the tensile force in the line is detected or estimated, for example, by detecting the energy or current that is drawn by an actuator. In addition, because load force is a dominating factor in establishing the magnitude of tensile force, load force can be used to roughly approximate tensile force and vice versa. Hereinafter, it should be understood that tensile force can be estimated using load force and load force can be estimated using tensile force. A signal representing the load force or tensile force on the line is sent to the controller, and the controller uses the load force or tensile force signal to drive the actuator effectively in response to the human input. This, for example, can prevent the actuator from releasing line when the load force or tensile force is zero so that although the line may become loose (i.e. not taut), slack (as defined above) will never be created in the line.
With this load sharing concept, the operator has the sense that he or she is lifting the load, but with far less force than would ordinarily be required. The force applied by the actuator takes into account both the gravitational and inertial forces that are necessary to move the load. Since the force applied by the actuator is automatically determined by line force and the force applied to the end-effector by the operator, there is no need to set or adjust the human power amplifier for loads having different weights. There is no switch, valve, keyboard, teach pendent or push-button in the human power amplifier to control the lifting speed of the load. Rather, the contact force between the human hand and the end-effector handle combined with line force are used to determine the lifting speed of the load. The human hand force is measured and used by the controller in combination with line force to assign the required angular speed of the pulley to either raise or lower the line and thus create sufficient mechanical strength to assist the operator in the lifting task. In this way, the device follows the human arm motions in a xe2x80x9cnaturalxe2x80x9d way. When the human uses this device to manipulate a load, a well-defined small portion of the total load force (gravity plus acceleration) is lifted by the human operator. This force gives the operator a sense of how much weight he/she is lifting. Conversely, when the operator does not apply any vertical force (upward or downward) to the end-effector handle, the actuator does not rotate the pulley at all, and the load hangs motionless from the pulley.
Although the existing devices described in earlier paragraphs do lift loads, they:
do not give the operator a physical sense of the lifting maneuver,
do not compensate for inertia forces,
do not compensate for varying loads,
do not address any key ergonomic concerns, and
do not prevent slack in the line.
The device of this application does have the above-identified advantages.